1. Field of the Invention
The present invention refers to a rotary hearth furnace for use in the iron and steel industry.
2. Description of Related Art
Rotary hearth furnaces have been used for a long time, particularly in the iron and steel sector.
Their uses are very varied. For example they are used to heat metals in ingots, slabs or blooms, before rolling; or for the heat treatment of materials, such as metal parts, glass or graphite; or again for processing loose or agglomerated raw materials such as coal or alumina, or mixtures of raw materials such as iron ore with carbon materials, or waste rich in iron with carbon materials.
Rotary hearth furnaces are built in different shapes and with a diameter varying from a few metres to more than 50 m, with width even larger than 6 m.
The hearth rotation speeds are also variable. Large heating furnaces rotate at even less than one revolution per hour, while small rotary furnaces, for calcination or for processing raw materials, reach for example fifteen revolutions per hour.
The hearth is generally in the shape of an annulus and rotates through two circumferential sets of wheels. These are located on the circumferences close to the ends of the annulus.
The wheels run on rails, and two different construction solutions are possible.
A first solution is to make wheels integral with the frame of the rotary hearth and rails fixed to the ground, mounted on very rigid structures, often made of reinforced concrete.
The second solution is to make rails integral with the frame of the rotary hearth and wheels fixed to the ground, mounted on very rigid structures, often made of reinforced concrete.
In the latter case the rotary hearth must be planned and built considering a fatigue stress in the metal structure, and consequently in its refractory lining, due to the continuous changing of the points of contact between the wheels and the rails during rotation of the hearth.
This fatigue stress may be very critical for the life of the refractory and so the furnaces are planned with wheels positioned on the two diameters, internal and external, on the same radii, so as to be able to divide the above metal structure into sectors having the same angular spacing as the wheels. In doing this, the deflection of the hearth structure due to the changing of the position of the point of contact of the wheels with the rails applied on the structure generates limited stresses and eliminates or minimises the cyclical movements of the refractory.
New processes have recently been developed in the field of the treatment of iron ore which require rotary hearth furnaces with very large hearth areas, and in some cases with very high rotating speeds, even more than 15 revolutions per hour.
These rotary hearth furnaces require hearth surfaces larger than those built up till now, with diameters even larger than 50 m and hearth widths larger than 6 m, even over 10 m.
In these furnaces certain problems, which are not important in the traditional applications, become critical when the dimensions and the rotating speeds are increased so considerably. The main problems to be tackled are the wear of the coupling between wheels and rails, and the curving of the hearth panels due to the difference in temperature in the panel supporting structure.
Normally the wheels and the rails are generally positioned on two circumferences very close to those of the ends of the hearth. Due to the geometry of the system, the wheels on the outer surface are more loaded than the wheels on the internal circumference. With the increase in the width of the hearth, this load difference is increased and consequently there may be great differences in the wear of the wheels and of the rails, which are on the two internal and external circumferences. The behaviour described above may be compensated, for example, by changing the size of the wheels and of the rails.
In these furnaces, also the planarity of the hearth is of primary importance. As is known, the supporting beams of the refractory hearth are subject to heating due to heat conduction through the hearth and at the same time they are cooled by irradiation and conduction with the environment below. This normally generates, in these supporting beams, a difference in temperature between the top and the bottom of the hearth, giving rise to a phenomenon of curving of the hearth when it reaches the working temperature.
When the width of the hearth increases, the traditional construction with frames having panels as wide as the hearth itself becomes critical on account of the strains due to said thermal effects and the consequent stresses or strains which may be generated in the refractory structure.
Moreover, since some new rotary hearth furnaces need to be installed at a considerable height above ground level, even higher than 20 metres, the high supports of the furnace, which must support the wheels or the rails of the hearth, increase the flexibility of the structure. In particular, if the structure, under the load of the hearth, generates different deflections from point to point, and particularly in an asymmetric manner, the induced stresses may be very critical, provoking functional complications and phenomena of fatigue during operation of the furnaces.